Power cables as presently installed either in the ground, in open conduits where air circulation varies or in the case of large cities in the proximity of underground steam pipes. All these conditions change the temperature profiles of the cable involved. Power cables as presently installed do not include a means for tracking the temperature of the cable conductor, particularly when subjected to overloads. Experience with underground power cables in particular indicate that overload conditions and therefore over temperature conditions can be so severe that the power cables themselves will ignite and combust. Similar experiences include power cables igniting in the walls of buildings when such cables are supplying high power equipment.
Electrical insulating materials having temperature-dependent resistance or capacitance characteristics have long been extensively used in overheat sensing and control applications. Thus, by virtue of the invention of Spooner and Greenhalgh disclosed and claimed in U.S. Pat. No. 2,581,212 overheat protection for electric blankets and similar articles has provided for the use of such materials to afford the essential safety factor. In accordance with the teachings of that patent, the insulating material is operatively associated with switch means and is coextensive with the heating element so that when the temperature anywhere in the blanket exceeds a predetermined maximum, the blanket heating power supply is interrupted. Because this insulating material is not altered physically or otherwise irreversibly changed in so functioning, it is useful repeatedly for this purpose as it acts as a sort of electrical switch constantly monitoring the blanket operating temperature limit.
Spooner, Jr. has further proposed, in U.S. Pat. No. 2,581,213, a temperature responsive signaling and locating system, basically in the form of a temperature sensitive cable, adapted to conventional junction box connections. The thermally-sensitive material in the structure causes operation of an alarm and/or indicating circuit before the thermally-sensitive material is permanently damaged. The Spooner '213 invention consists of three conductors, the third of relatively low resistance, arranged in a voltage measuring circuit. When a sufficiently high temperature is reached at any point along the cable, conduction of current through the thermally-sensitive material separating the first conductors completes a measuring circuit, for example, by deflection of a needle or activation of a bell or other alarm. This is indicative of an abnormal heat condition in the area being monitored.
The over temperature problems associated with power cables have not heretofore been addressed and as discussed above are unique.
It has become apparent from recent underground cable fires that there is a need for such an apparatus which is capable of monitoring, detecting, alarming and otherwise responding to over temperature conditions in power cables along the entire length of the cable.